The present invention generally relates to compositions which are curable by two different mechanisms. More particularly, the present invention relates to compositions which cure by both a free radical photoinitiated crosslinking reaction and a platinum catalyzed thermal hydrosilation reaction.
Silicone compositions have become widely accepted as protective coatings for electronic components mounted on circuit boards. The moisture resistance, thermal stability and resistivity of silicones make them ideal for this purpose. Previously developed silicone conformal coatings have been heat curable and furnished in organic solvents, however high energy costs as well as safety and environmental considerations have led to the development of such alternate technologies as ultraviolet radiation curable silicone coatings.
Both heat curable and UV-curable silicone conformal coatings have certain disadvantages. The use of heat curable conformal coatings risks damaging many fragile, heat sensitive electronic components. Consequently, thermal cure cycles can be extremely long, which in turn reduces production of coated circuit boards. On the other hand, while radiation curable silicone conformal coatings speed processing and avoid damaging the electronic components, in many instances a complete cure of the conformal coating is not effected due to what is called the "shadow effect". Briefly, the shadow effect is caused by components mounted on circuit boards which, because they project off the surface of the board, cast shadows and thereby prevent effective curing. While persons skilled in the art can overcome much of the shadow effect with mirrors, there can be crevices and the like where ultraviolet light simply cannot penetrate and thus where UV-curable compositions cannot be cured.
In view of the foregoing, the skilled artisan will appreciate that it is highly desirable to have available a composition which is both UV-curable and thermally curable (either at room temperature or at elevated temperatures) in order to overcome such shortcomings.
It is also desirable that such compositions can be utilized in other applications, for example, as silicone release coatings.
Platinum catalyzed addition curable silicone compositions are well known in the art. For example, Grenoble in U.S. Pat. No. 3,900,617 disclosed that flexible sheet material can be rendered non-adherent to surfaces which normally adhere thereto by coating the sheet material with a coating composition formed from (1) a vinyl chainstopped polysiloxane, (2) a hydrogen-containing polysiloxane and (3) a platinum catalyst effective to cause copolymerization of (1) and (2).
Other variations of such technology are disclosed in U.S. Pat. Nos. 4,256,870 and 4,340,647 to Eckberg, both of which are assigned to the same assignee as the present invention and are incorporated by reference into the instant disclosure.
Ohto et al., U.S. Pat. No. 3,865,588, discloses photopolymerizable compositions which contain at least one organopolysiloxane having an unsaturated radical of the formula ##STR1## where R.sup.1 is a hydrogen atom, a phenyl radical or a halogen substituted phenyl radical and R.sup.2 is a hydrogen atom or a methyl radical. Exemplary of such radicals are the acryloxy radical, methacryloxy radical, cinnamoyloxy radical or a halogenated cinnamoyloxy radical.
In U.S. Pat. No. 3,726,710 to Berger et al. there is disclosed a composition comprising a vinyl group containing polyorganosiloxane having a sensitizer added thereto and which is curable by exposing it to high intensity ultraviolet rays.
Hatanaka et al., U.S. Pat. No. 4,451,634, discloses silicone elastomeric compositions suitable for ultraviolet ray curing comprising (A) 100 parts by weight of a polyorganosiloxane of the general formula EQU (CH.sub.3).sub.2 R.sup.1 SiO(R.sub.2.sup.2 SiO).sub.n SiR.sup.1 (CH.sub.3).sub.2
wherein R.sup.1 represents hydrogen or a monovalent radical selected from methyl, vinyl and hydroxy, R.sup.2 represents hydrogen or a substituted or unsubstituted monovalent hydrocarbon radical, 0.02-49.95% based on the total number of R.sup.1 and R.sup.2 groups are vinyl radicals, 0.05-49.98% based on the total number of R.sup.1 and R.sup.2 groups are hydrogen, and n represents a number from 10 to 10,000; (B) 0.5 to 10 parts by weight of a polyorganosiloxane having the formula ##STR2## wherein R.sup.3 is a substituted or unsubstituted monovalent hydrocarbon radical other than vinyl, a represents a number of 0.01.ltoreq.a.ltoreq.1 on average, b represents a number of 0.ltoreq.b.ltoreq.3 on average, and a+b is a number from 1 to 3; and (C) 0.1 to 10 parts by weight of a peroxy ester of the formula ##STR3## wherein R.sup.4 is a substituted or unsubstituted monovalent aliphatic radical and R.sup.5 is a substituted or unsubstituted monovalent aromatic radical.
Eckberg et al., U.S. patent application Ser. No. 527,299, filed Aug. 26, 1983, and assigned to the same assignee as the present invention, discloses an ultraviolet radiation curable composition comprising: (A) a diorganopolysilioxane comprising units of the formula RR'SiO, wherein R is hydrogen or a C.sub.1-8 alkyl radical and R' is hydrogen, a C.sub.1-8 alkyl radical or a monovalent mercaptoalkoxyalkyl-functional organic radical of 2 to 20 carbon atoms, (B) a polysiloxane consisting of from 0.5 to 100 mole percent of vinyl functional siloxane units of the formula (CH.sub.2 =CH)R.sub.n SiO.sub.(3-n)/2, where R is hydrogen or a C.sub.1-8 alkyl radical and n has a value of 0 to 2, inclusive, and (C) a catalytic amount of photoinitiator.
Nowhere, however, to applicant's knowledge, is there disclosed or suggested a silicone composition which cures by both a free radical catalyzed UV-hydrosilation reaction and a platinum catalyzed hydrosilation reaction, and thereby avoids the shortcomings of those compositions which cure by only one of the recited mechanisms.